overview: Ignore dragEnd while the animation is still in progress

Moving the mouse fast enough during xdnd will trigger a xdnd-leave event
because the input shape is not updated until after the animation is done.

So simply ignore the leave events while the animation is in progress.

https://bugzilla.gnome.org/show_bug.cgi?id=708887

js/ui/overview.js

@@ -313,6 +313,9 @@ const Overview = new Lang.Class({
     },
 
     _onDragEnd: function(time) {
+        if (this.animationInProgress)
+            return;
+
         this._inXdndDrag = false;
 
         // In case the drag was canceled while in the overview